Make the first cake layer. Position a rack in the lower third of the oven and preheat to 350 degrees . Butter a 12-inch square 2- to 3-inch-high cake pan. Line with parchment and butter the parchment.
In a 6-quart pot, bring 4 cups water and 4 cups granulated sugar to a boil, whisking occasionally, until the sugar is dissolved. Remove from the heat and add 12 tablespoons butter and half of the chocolate. Let cool for about 20 minutes, whisking occasionally, until the chocolate is melted.
Meanwhile, in a large bowl, whisk together 3 1/2 cups flour, 1 tablespoon plus 1 teaspoon baking powder, 2 teaspoons baking soda and 1 teaspoon salt.
Whisk 1 1/2 cups cocoa powder into the chocolate mixture (it will still be warm). In a small bowl, beat together 4 eggs with a fork, then whisk into the chocolate mixture with 1 tablespoon vanilla. Let the mixture stand until cool to the touch, about 10 minutes. Whisk in the flour mixture until smooth. Pour the batter into the prepared pan and bake until a cake tester inserted in the center comes out clean, about 50 minutes. Let the cake cool in the pan on a rack for 20 minutes, then cut around the edges, invert onto a baking sheet and remove the pan. Invert again (right side up) onto a rack to cool completely.
To make the second layer, repeat with the same (clean) pan, using the remaining cake layer ingredients. You can prepare the cake layers 1 day ahead before frosting: Wrap well in plastic wrap and keep at room temperature.
Make the filling. Using an electric mixer, beat the butter, peanut butter, cream cheese and salt at medium speed until smooth. Mix in the confectioners sugar at low speed until fluffy. Mix in the chocolate chips.
Make the frosting. In a large bowl, whisk together the confectioners sugar and cocoa powder. Using an electric mixer, cream the butter, cream cheese and salt at high speed until fluffy. Mix in the cocoa-sugar mixture 1 cup at a time at low speed. Beat in the vanilla at high speed until fluffy.
Assemble the cake. If the cake layers are domed in the center, trim them flat using a serrated knife (the cake scraps are good for snackin!). Place a cake layer right side up on a 12-inch square cake board. Spread the peanut butter filling evenly on top with an offset spatula. Invert the second cake layer (upside down) onto a flat cookie sheet, then slide it carefully on top of the filling, so that the edges of the 2 layers are aligned. Frost the top and sides with the chocolate frosting. Decorate with the nonpareils.
The Do's and Don'ts of Cooking with Chocolate